1. Field of the Invention
The present invention relates to a transparent plate, a process for the production thereof and a transparent plate-applied screen display plate, and a Braun tube (cathode ray tube) and a process for the production thereof. In particular, it relates to a transparent plate using ultrafine particles capable of effectively functioning as an antistatic and anti-reflection film for a screen display plate, a process for producing the same and a transparent plate-applied screen display plate, and a Braun tube (cathode ray tube) and a process for the production thereof.
2. Related Art
Films to reduce the reflectance of a transparent plate surface have been long studied, and have been applied to lenses for cameras and ophthalmic glasses. At present, such films are used as an anti-reflection filter for reducing the reflected light on VDTs (visual display terminal). A variety of anti-reflection films have been considered, and mainly used now are multi-layered films and heterogeneous films.
A multi-layered film has a structure in which a material having a low reflectance and a material having a high reflectance are alternately stacked to form at least three layers. Its anti-reflection effect is a synergistic effect produced by the optical interference function of each layer. Multi-layered films are discussed in Physics of Thin Films, 2 (1964), pp. 243-284.
A heterogeneous film having a reflectance distribution in the film thickness direction works as an anti-reflection when the film has a lower refractive index than a glass base plate. A heterogeneous film is generally formed by rendering a transparent plate surface porous.
Apl. Phys. Lett., 36 (1980), pp. 727-730 discusses a method of reducing the reflectance in which a heterogeneous film is produced by forming an insular metal deposition film on a glass surface and forming a fine uneven surface by sputter etching.
Solar Energy 6 (1980), pp. 28-34 discusses a method of reducing the reflectance in which a soda glass surface is rendered porous by dipping it in an H.sub.2 SiF.sub.6 solution oversaturated with SiO.sub.2.
On the other hand, in a cathode ray tube, it is required not only to form an electrically conductive film for prevention of electrostatic charge but also to use devices for prevention of reflection.
Meanwhile, it is known that the front panel surface (image display plate) of a cathode ray tube such as a Braun tube is electrostatically charged. The reason therefor is as follows. Aluminum is generally deposited to form a thin and uniform film on a phosphor 43 applied to an inner surface 42 of a Braun tube 41 as shown in FIG. 4. In the application of a high voltage to the aluminum film 44, an electrostatic charge occurs on a front panel 45 of the Braun tube due to electrostatic induction when the high voltage is applied and cut off.
JP-A-61-51101 discloses a method of forming an antistatic and anti-reflection film for prevention of both electrostatic charge and reflection on such a display tube surface. In this method, first, an electrically conductive film is formed on a glass base plate by a physical gas phase method or a chemical gas phase method such as a vacuum deposition method and a sputtering method, and then, an anti-reflection film is formed thereon.
In the above prior arts, the film forming method is limited to a sputtering or vacuum deposition method, and it is required to control the film thickness highly accurately. There is therefore a defect in that a high cost is required and it is difficult to apply these prior art methods to a base plate having a large surface area.
In particular, the above prior art method uses a two-layer structure, in which an electrically conductive film and an anti-reflection film are formed, respectively. There have been therefore problems in productivity and cost. Further, when such films are formed on the surface of a display tube such as a Braun tube which limits the firing temperature for forming films to low temperatures, there have been problems in film strength and reflectivity.
In a reflection film containing ultrafine particles, a minimum reflectance is obtained when the ultrafine particles are highly densely and regularly arranged on a base plate.
FIG. 5 schematically shows a cross-sectional view of a film in which ultrafine particles are systematically and regularly applied to a transparent base plate. In FIG. 5, numeral 46 indicates ultrafine particles, numeral 47 indicates a binder layer, and numeral 48 indicates a base plate. In this Figure, n.sub.0 is the refractive index of air, n.sub.1 is the refractive index of an ultrafine particle layer, da, on the air side, n.sub.2 is the refractive index of the da layer on the ultrafine particle side, n.sub.S is the refractive index of a layer formed from ultrafine particles and a binder, and n.sub.G is the refractive index of the transparent base plate. In this case, the reflectance, Ra, of the da layer is represented by the equation (expression 1), and the reflective index, Rb, of the db layer, by the equation (expression 2). ##EQU1##
When the reflectance of a portion where no ultrafine particles are present is taken as Rc, the total reflectance is represented by the equation (expression 3), EQU R=(1-.alpha.)(Ra+Rb)+Rc (Expression 3)
in which .alpha. is the ratio of an area where no ultrafine particles are present.
When the binder is a vitreous binder, Rc is generally 4.2%.
Ra is about 0.19% at .lambda.=550 nm on the assumption that n.sub.0 =1.0, n.sub.1 =1.10, n.sub.2 =1.38 and n.sub.S =1.47. When the transparent plate is glass, Rb is about 0.04% at .lambda.=550 nm on the assumption that n.sub.G =1.53 and that the other refractive indexes are the same as those in Ra.
The consequence is (Ra+Rb)&lt;Rc. That is, it is understood that the smaller .alpha. is, the smaller the reflectance is. In other words, when ultrafine particles are regularly and densely applied, the reflectance is the lowest.
The present inventors have already proposed the application of ultrafine particles to an anti-reflection film and filed on Sep. 8, 1989 as U.S. Ser. No. 07/404553 (U.S. Pat. No. 5,189,337) whose content is incorporated herein by reference. As a result of a further study, it has been also found that a coating solution level is elevated up or down on the base plate surface at a constant rate, whereby ultrafine particles contained in the coating solution are regularly arranged on, and applied to, the base plate surface to give a low reflectance close to a theoretical value.
It has been also found that, in the above case, ultrafine particles having an uneven surface are used, whereby there is obtained a film which shows a decrease in diffuse reflection and is not opacified.
It has been further found that, in the above case, antistatic ultrafine particles of which the diameter is not more than 1/10 of that of the anti-reflection ultrafine particles are incorporated, whereby antistatic ultrafine particles are arranged in a network form in gaps among the anti-reflection ultrafine particles to form an electrically conductive film.
The present invention provides an antistatic and anti-reflection film which can be applied to a large area at a low cost and an image display to which the film is applied.
The present invention is achieved either by filling a coating solution containing uniformly dispersed ultrafine particles in a bath positioned on the side of a surface of a base plate and pulling up the base plate at a constant rate, or by filling the above coating solution in a bath positioned on the side of a surface of a base plate at a constant rate.
The present invention is achieved by making uneven surfaces of anti-reflection ultrafine particles, or by at least making the surfaces porous. Otherwise, uneven portions may be formed by aggregating ultrafine particles to form fine particles which have gaps formed of the ultrafine particles on surfaces of the fine particles.
The process for producing a transparent plate, provided by the present invention, has a characteristic feature in that a film composed of ultrafine particles and a binder to be filled in gaps among said particles is formed on a transparent base plate by resting the base plate against a container, introducing a mixed coating solution containing said particles and said binder into the container to elevate the mixed coating solution level up on the base plate surface at a constant rate, or lowering the mixed coating solution level on the base plate surface at a constant rate, to form the ultrafine particle film on the base plate surface.
The transparent plate of the present invention has the following characteristic features. In a transparent plate obtained by forming an ultrafine particle film of ultrafine particles and a binder filled in gaps among the ultrafine particles on a transparent base plate, a coupling agent having a functional group to a material of the base plate is incorporated into the ultrafine particle film and/or allowed to present in the interface between the ultrafine particle film and the transparent base plate, and/or a coating liquid composed mainly of ethyl silicate is further applied onto a coating of the ultrafine particles.
The image display plate and the image display protection plate provided by the present invention have a characteristic feature in that the above-prepared transparent plate is applied to the surface of the transparent base plate or that the ultrafine particle film is directly formed on the image display plate surface.
The Braun tube of the present invention has a characteristic feature in that it is formed from the above image display plate or the above image display protection plate. In addition, the present invention can be also applied to other cathode ray tubes, liquid crystal display devices, window glass for automobiles and ophthalmic glasses, and the like.
The process for producing the Braun tube, provided by the present invention, has a characteristic feature in that the transparent base plate surface of the Braun tube is exposed through an opening portion provided on the side of a container, and a mixed solution containing ultrafine particles and a binder is introduced into the container to elevate the mixed coating solution level up on the base plate surface at a constant rate, or lowering the mixed coating solution level on the base plate surface at a constant rate, to form an ultrafine particle film on the base plate surface.
The process for producing the window glass for an automobile has a characteristic feature in that the transparent base plate surface of the window glass for an automobile is exposed through an opening portion provided on the side of a container, and a mixed solution containing ultrafine particles and a binder is introduced into the container to elevate the mixed coating solution level up on the base plate surface at a constant rate, or lowering the mixed coating solution level on the base plate surface at a constant rate, to form an ultrafine particle film on the base plate surface.